1. Field of the Invention
The present invention relates to propulsion systems for motor vehicles, and more particularly to electrically-propelled vehicles.
2. Description of the Related Art
In an electric vehicle which employs a fuel cell for its electric power, a concern arises regarding start-up of the fuel cell. Typically, reaction air is provided to the fuel cell by a turbocompressor driven in part by exhaust gases from the fuel cell's operation and in part by an electric motor. For optimal efficiency and reduced packaging size, it is advantageous for the compressor's motor to be a high-voltage device coupled to the relatively-high voltage output of the fuel cell. However, when the fuel cell is to undergo start-up, the fuel cell is not yet providing electrical energy or exhaust gas sufficient to operate the compressor. Without a compressor operating at sufficient capacity, the fuel cell will not have reaction air sufficient for start-up.
Several methods for providing reaction air to the fuel cell might be tried. For example, a supply of compressed air could be carried onboard the vehicle.
That compressed air could be used to start the fuel cell. However, such a supply of compressed air would have disadvantages regarding packaging in the vehicle.
Another method for providing reaction air at startup would be to provide high-voltage electrical energy storage on the high voltage bus of the vehicle (where the compressor would also most advantageously reside). Such stored electrical energy could power the compressor for start-up. However, providing high voltage storage for this singular purpose would be disadvantageous both for cost and packaging.
A typical vehicle having a high-voltage drive portion, such as a vehicle powered by a fuel cell, will still have a 12-volt portion which includes the traditional electrical accessories on a vehicle. This 12-volt portion would have a battery for energy storage and would be supplied electrical energy by a DC-to-DC downconverter from the high voltage bus. Yet another method for providing reaction air at start-up would be through the use of a DC-to-DC upconverter which would convert voltage from the 12-volt battery to high voltage for the compressor at start-up. After a very short start-up period (typically only about 20 seconds), this upconverter would not be used. Although workable, this is a cost- and packaging-inefficient use of the additional hardware represented by the DC-to-DC upconverter.
Therefore, a design which will overcome start-up difficulties with fuel-cell-propelled vehicles and which will do so in a cost-effective and packaging efficient manner will provide advantages over alternative designs.